1. Field of the Invention
The present invention relates to a piezoelectric composite material and more particularly to a piezoelectric composite material comprising a ferroelectric ceramic powder and a polymer.
2. Description of the Prior Art
In general, attempts to obtain piezoelectric materials by compounding polymers and ferroelectric ceramic powders have been known [Mat. Res. Bull., Vol. 13, pp. 599-607 (1978), Pergamon Press Inc. (U.S.A.)]. The ferroelectric ceramic powders have been prepared by carrying out a solid phase reaction under applied heat in the case of preparing titanium-containing ferroelectric ceramics such as BaTiO.sub.3, PbTiO.sub.3, PbZrO.sub.3 --PbTiO.sub.3 solid solutions and the like; or preparing single crystals in the case of potassium sodium niobate (PSN) and the like, and grinding the product obtained to powders using a grinder such as a ball mill, a vibrational mill or the like until the powders have the desired size distribution. However, ferroelectric ceramic powders prepared by conventional grinding processes are attended by conspicuous difficulties when used in a composite with polymers. That is to say, they barely have even low efficiency which is quite contrary to what one might expect based upon the piezoelectric characteristics of the ferroelectric ceramics themselves, and composite materials of this type are fragile and hard to mold because they lack pliability, moldings thereof are heavy, expensive, etc. As a result, composite materials of this type have not reached the stage of development at which their use is practical.
In the conventional grinding process carried out after the solid phase reaction or the preparation of single crystals, structural fractures arise in the microcrystals of the ceramic which result in the formation of numerous domains or distorted phases. The resulting multiple domains or distorted phases bring on the following problems when the microcrystals are compounded with a polymer. Namely, it becomes almost impossible to force all the resulting numerous multiple domains or distorted phases to line up in the same direction as that of the externally applied electric field even at a high voltage close to the maximum voltage the composite material can resist without dielectric breakdown. The orienting electric field is markedly reduced to several tenths or several hundredths the electric field in the compounding state taking into account the ratio of the dielectric constant of the polymer to that of the ceramics. Thus, admixing of ceramic powders or single crystals with polymers cannot impart piezoelectric properties to an appreciable extent to the resulting composite materials and, consequently, it has scarcely any technical significance.